So far, there have been available solar heat collectors of the type wherein the open ends of a plurality of heat collecting tubes terminate in the lower portion of the reservoir. Such collectors are also designed such that the heat collecting tubes inserted into the reservoir are provided in the sides with inflow openings, through which cold water on the bottom of the reservoir flows into said tubes, whereby a spontaneous convection takes place between cold water and hot water.
However, such a type of heat collector has the following disadvantages. Since the open ends of the heat collecting tubes are located within the reservoir, heated hot water circulates by convection to an insufficient extent, so that it remains substantially stagnant within the reservoir. In some cases, an extremely insufficient convection may take place depending upon the position at which the tubes are attached to the reservoir. In addition, the narrow system for heat circulation considerably deteriorates heat conduction. Furthermore, cold water on the bottom of the reservoir flows into the heat collecting tubes through the inflow openings, resulting in considerable energy loss. Thus, the conventional spontaneous convection type solar heat collector arrangements are still unsatisfactory in several points.